Efficient coupling of a light source into a thin film optical waveguide is a problem for integrated optics devices. The difficulty is maintaining efficient coupling of the light into the waveguide with minimal alignment difficulties. Grating couplers and taper couplers have been utilized for this purpose. Prior grating couplers utilized a grating depth variation to increase the theoretical coupling efficiency.
A grating coupler couples the light by diffracting an incident beam off a grating on the surface of the waveguide either into or out of the planar waveguide, depending on whether it is used as an input or an output coupler. The main disadvantage of the grating coupler is that efficient coupling is obtained only over a narrow range of wavelength and/or incidence angles. In current optical diode laser applications, it is difficult to use grating couplers because of beam divergence and wavelength instability. Accordingly, it will be appreciated that it would be highly desirable to have an optical diode laser structure wherein it is easy to use grating couplers, and wherein the difficulties with beam divergence and wavelength instability are eliminated.
A taper coupler couples the light either into or out of the film by total internal reflection of the beam off of a taper in the film waveguide. The reflection of a propagating beam in the taper changes the propagation angle in the waveguide, either changing an input beam toward the mode propagation angle within the waveguide, or changing a mode propagating in the waveguide. The mode propagating in the waveguide is changed to an angle less than that for total internal reflection at the waveguide/substrate interface, and the beam is then outcoupled into the substrate. A characteristic of this coupler is that the beam outcoupled from the waveguide is spread over a wide angular range of 10.degree. or more, compared to the grating input coupler which is spread over an angular range of 1.degree. or less. The main problem for input coupling is mode matching, which is very difficult to achieve due to a complex relationship between beam divergence and the intensity distribution. A difficulty for output coupling is that the outcoupled beam cannot be collimated or imaged clearly, therefore, it is difficult to utilize the beam for functions other than simple intensity detection. Accordingly, it will be appreciated that it would be highly desirable to have a structure wherein mode matching can be achieved and the outcoupled beam can be collimated or imaged clearly.